The invention herein pertains generally to digital communications and specifically to the establishment of proper symbol timing requisite to the detection of baud values where the data bauds are conveyed by AC carrier signal.
The transmission of digital data by varying some characteristic of an AC carrier signal, such as its phase or frequency, commonly referred to as phase shift and frequency shift keying, respectively, is widely practiced. Intrinsic to this mode of communications is the need to establish appropriate symbol (also referred to as baud) timing at the receiver to identify the beginning and termination of each baud period, wherein a single unit of data comprising one or more data bits is transmitted, to permit the detection thereof. As is well documented in the technical literature, including U.S. Pat. No. 3,368,036 entitled "Demultiplexing and Detecting System for Predicted Wave Phase Pulse Data Transmission System" owned by the same assignee herein, a common technique to detect the value of each data baud is one known as "integrate and dump" wherein a signal is allowed to linearly vary over the baud period with a polarity which is a function of the carrier signal characteristic vis-a-vis some reference signal, thereby defining the baud value. The efficacy of this detection technique is based on encompassing each integration period within the associated transmitted baud period so that it does not overlap into the next baud period, and consequently the need for developing appropriate symbol timing in the receiver.
One conventional method for affording appropriate symbol timing is to transmit along with the carrier signal a pilot tone so that the receiver timing can be synchronized to that employed in the transmitter. In some communications media, however, this tone and its associated hardware can be eliminated by employing a synchronizing signal which is intrinsically available at both the transmitter and receiver. For example, in the evolving technical field of power line communications for permitting electric utility companies to transmit communication signals over their distribution power lines to remotely control customer loads and monitor energy consumption, symbol timing can be derived from the 60 hertz power system itself since the transmitter and receiver are both connected thereto. Relying on the 60 hertz power signal as a synchronizing agent, however, creates a problem in selecting the proper cycle during a given baud period as well as the point of the AC cycle to which to synchronize. Since the zero crossing of an AC signal is the most discernible and therefore most logical point to which to synchronize, this is the common practice. However, ambiguities arise, the severity of which is dependent on the baud rate. As an example, if one were to transmit at a data rate of 60 baud/second, there would be one 60 hertz AC cycle and concomitantly two zero crossings per baud between which to choose for synchronization. At a lower rate, for instance 20 baud/second, there would be three 60 hertz AC cycles and concomitantly six zero crossings per baud to contend with. If the integration period corresponding to an individual baud were to be initiated on the wrong zero crossing, it then would overlap into the consecutive baud period, rendering it more difficult to accurately detect data by adding to or subtracting from the integrated value which would have been obtained if the integration had been initiated at the right zero crossing.
In view of the foregoing, it is an object of the present invention to provide a new and improved means for deriving symbol timing from an available AC signal for detecting digital data conveyed by the varying characteristic of a carrier signal where symbol timing itself is synchronized to the AC signal.
It is a further object of the present invention to provide such a new and improved means which obviates ambiguities encountered when deriving the symbol timing from the AC signal having multiple zero crossings during each baud period.
It is still a further object of the present invention to provide such a new and improved means which may be digitally implemented so as to realize the economic benefits of large scale integration techniques.
The foregoing objects, as well as others, and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Preferred Embodiment which follows together with the appended drawings.